Backup members are members that only receive traffic when all non-backup members are down. The API, CLI, SDK, Dashboard and Tempest API work has been completed. What is missing is Tempest scenario testing and documentation.
Existing scenario tests cover backup members already: https://opendev.org/openstack/octavia-tempest-plugin/src/branch/master/octavia_tempest_plugin/tests/scenario/v2/test_member.py#L479.